Threaded joints are widely used for connecting OCTG and other steel pipes used in oil industry facilities. Typically, standard threaded joints as specified in American Petroleum Institute (API) standards have been used to connect steel pipes that are used to prospect for and to produce oil and gas. However, the drilling/production environment is becoming increasingly severe in recent years because crude oil wells and natural gas wells are becoming deeper and the number of horizontal wells and directional wells are increasing relative to the number of vertical wells. Because development of wells in hostile environments such as oceans and polar regions is increasing, various characteristics such as resistance to compression, resistance to bending, sealability against external pressure (external pressure resistance) are required for a threaded joint. Therefore, an increasing number of special high-performance threaded joints, which are called premium joints, are being used, and the demand for improving the characteristics of such joints has been increasing.
Typically, a premium joint is a coupling-type joint in which a pair of externally threaded members (hereinafter referred to as pins) formed at ends of pipes and an internally threaded member (hereinafter referred to as a box) are coupled. Each of the pins includes a tapered thread, a seal portion (to be specific, a metal-to-metal seal portion), and a shoulder portion (to be specific, a torque shoulder portion). The box connects the pins to each other. The tapered thread is important for strongly fixing the pipe joint. The seal portion serves to secure sealability because the box and the pins come into metal-to-metal contact with each other in this portion. The shoulder portion forms a shoulder face (bearing face) that serves as an abutment when the joint is being fastened.
FIG. 4 schematically illustrates a premium joint for OCTG and shows a longitudinal section of a threaded joint for circular pipe. The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. Each of the pins 3 includes an externally threaded portion 7 formed on an outer face thereof and a nose 8 (pin nose 8). The nose 8 is an unthreaded portion disposed at an end of the pin 3 and is adjacent to the externally threaded portion 7. The nose 8 includes a seal portion 11 on an outer peripheral face thereof and a torque shoulder portion 12 on an end face thereof. The box 1, which faces the pin 3, includes an internally threaded portion 5, a seal portion 13, and a shoulder portion 14, which are respectively capable of mating with or coming into contact with the externally threaded portion 7, the seal portion 11, and the shoulder portion 12 of the pin 3.
Patent Literatures 1 to 3 describe existing technologies related to the premium joints.
In the example illustrated in FIG. 4, the metal-to-metal seal portion is disposed at the end of the pin nose 8. Patent Literature 1 describes a threaded joint in which a metal-to-metal seal portion is disposed near a threaded portion of the pin nose 8 and the nose is elongated so as to extend from the seal portion to the shoulder portion so that the external pressure resistance is increased. In the threaded joint described in Patent Literature 1, the pin nose that is not in contact with a box member is elongated so as to form a shape that is discontinuous with the seal portion so that the thickness of the pin nose is not reduced. As a result, not only improvement in the external pressure resistance but also improvement in the resistance to axial compression are realized.
Patent Literature 2 describes a threaded joint that includes an appendix that extends from a seal portion to an end of a pin nose and that has a shape that is discontinuous with the seal portion. With the appendix, the rigidity in the radial direction is secured while the rigidity in the axial direction is reduced. The appendix becomes deformed when the threaded joint is fastened and recovers its original shape when a tensile load is applied, and thereby the tension resistance is improved.
As described in Patent Literatures 1 and 2, disposing the seal portion near the threaded portion of the pin to separate the seal portion from the end of the pin nose is effective not only in increasing the external pressure resistance and the tension resistance but also in providing the thread with stable characteristics. This can be verified through a finite element method simulation or the like. Moreover, when a strong axial compression is applied to a pin nose having a shape that is discontinuous with a seal portion, the pin nose itself becomes deformed and thereby reduces the amount of plastic deformation of a torque shoulder portion of a box member. On the other hand, a discontinuous portion between the thread and the nose may become excessively deformed. It is supposed that the occurrence of such deformation depends on a make up torque.
A make up torque is influenced by the lubrication condition, the face condition, and the like. Examples of designs that are not highly dependent on such conditions include a radial seal design with which the radial component of the seal contact pressure is relatively increased and the seal contact pressure in the radial direction is increased. For example, Patent Literature 3 describes an example of the radial direction seal design in which a pin seal has a round shape with a large diameter and a small seal taper angle. However, the radial seal design having a small seal taper angle has a problem in that galling is likely to occur when the threaded joint is fastened. In particular, when it is necessary to provide a large amount of interference of the seal in order to secure the sealability and the stability of sealing, galling is more likely to occur.